linux-kernel - Re: [PATCH] writeback: call writeback tracepoints withoud holding list_lock in wb

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Date:	Thu, 25 Feb 2016 15:47:20 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	tj@...nel.org, jack@...e.cz, axboe@...com, fengguang.wu@...el.com,
	tglx@...utronix.de, bigeasy@...utronix.de,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-rt-users@...r.kernel.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH] writeback: call writeback tracepoints withoud holding
 list_lock in wb_writeback()

On 2/25/2016 3:31 PM, Steven Rostedt wrote:
> On Thu, 25 Feb 2016 15:16:54 -0800
> "Shi, Yang" <yang.shi@...aro.org> wrote:
>
>
>> Actually, regardless whether this is the right fix for the splat, it
>> makes me be wondering if the spin lock which protects the whole for loop
>> is really necessary. It sounds feasible to move it into the for loop and
>> just protect the necessary area.
>
> That's a separate issue, which may have its own merits that should be
> decided by the writeback folks.

Yes, definitely. I will rework my commit log for this part.

>
>>
>>>
>>>>
>>>>>
>>>>>
>>>>>> INFO: lockdep is turned off.
>>>>>> Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830
>>>
>>> Can you disassemble the vmlinux file to see exactly where that call is.
>>> I use gdb to find the right locations.
>>>
>>>    gdb> li *0xffffffc000374a5c
>>>    gdb> disass 0xffffffc000374a5c
>>
>> I use gdb to get the code too.
>>
>> It does point to the spin_lock.
>>
>> (gdb) list *0xffffffc000374a5c
>> 0xffffffc000374a5c is in wb_writeback (fs/fs-writeback.c:1621).
>> 1616
>> 1617            oldest_jif = jiffies;
>> 1618            work->older_than_this = &oldest_jif;
>> 1619
>> 1620            blk_start_plug(&plug);
>> 1621            spin_lock(&wb->list_lock);
>> 1622            for (;;) {
>> 1623                    /*
>> 1624                     * Stop writeback when nr_pages has been consumed
>> 1625                     */
>>
>>
>> The disassemble:
>>      0xffffffc000374a58 <+232>:   bl      0xffffffc0001300b0
>
> The above is the place it recorded. But I just realized, this isn't the
> issue. I know where the problem is.
>
>
>> <migrate_disable>
>>      0xffffffc000374a5c <+236>:   mov     x0, x22
>>      0xffffffc000374a60 <+240>:   bl      0xffffffc000d5d518 <rt_spin_lock>
>>
>>>
>
>
>
>>>> DECLARE_EVENT_CLASS(writeback_work_class,
>>>>            TP_PROTO(struct bdi_writeback *wb, struct wb_writeback_work *work),
>>>>            TP_ARGS(wb, work),
>>>>            TP_STRUCT__entry(
>>>>                    __array(char, name, 32)
>>>>                    __field(long, nr_pages)
>>>>                    __field(dev_t, sb_dev)
>>>>                    __field(int, sync_mode)
>>>>                    __field(int, for_kupdate)
>>>>                    __field(int, range_cyclic)
>>>>                    __field(int, for_background)
>>>>                    __field(int, reason)
>>>>                    __dynamic_array(char, cgroup, __trace_wb_cgroup_size(wb))
>>>>
>>>
>>> Ah, thanks for pointing that out. I missed that.
>>
>> It sounds not correct if tracepoint doesn't allow sleep.
>>
>> I considered to change sleeping lock to raw lock in kernfs_* functions,
>> but it sounds not reasonable since they are used heavily by cgroup.
>
> It is the kernfs_* that can't sleep. Tracepoints use
> rcu_read_lock_sched_notrace(), which disables preemption, and not only
> that, hides itself from lockdep as the last place to disable preemption.

Ah, thanks for pointing out this.

>
> Is there a way to not use the kernfs_* function? At least for -rt?

I'm not quite sure if there is straightforward replacement. However, I'm 
wondering if lock free version could be used by tracing.

For example, create __kernfs_path_len which doesn't acquire any lock for 
writeback tracing as long as there is not any race condition.

At least we could rule out preemption.

Thanks,
Yang

>
> -- Steve
>